Fuel assemblies for nuclear reactors, and particularly for pressurized water nuclear reactors consist of a bundle of fuel rods of great length which are held in a framework in such a way that the rods are parallel to one another and are arranged in a regular network in the transverse planes of the assembly perpendicular to its longitudinal direction.
The fuel rods are held by spacer grids uniformly spaced over the length of the assembly and forming elements of the framework.
The fuel assemblies of the pressurized-water nuclear reactors in operation at the present time have a length of over four meters and a square cross-section of the order centimeters on each side nuclear reactors having a power of 900 MW comprise eight spacer grids uniformly spaced over the length of the assembly, and fuel assemblies intended for reactors having a power of 1300 MW comprise ten spacer grids. The outer lateral faces of the spacer grids project slightly in relation to the bundle of fuel rods of the assembly.
To carry out the transport of fuel assemblies, for example new fuel assemblies intended for refuelling the core of a nuclear reactor, containers, in which two fuel assemblies are arranged and locked during transport, are employed.
The transport containers comprise lower and upper half-shells which can be assembled together by screwing and of which the lower half-shell receives an underframe or cradle for supporting the fuel assemblies and of which the upper half-shell forms the cover of the container.
The underframe supporting the fuel assemblies consists of an elongate structure, the T-shaped cross-section of which provides, for each of the fuel assemblies, two right-angled bearing surfaces arranged on either side of the middle part of the cradle forming a partition wall for the two assemblies.
The cradle rests within the lower half-shell of the container by means of shock-absorbing studs.
Each of the assemblies arranged in the container comes to bear against the cradle with two successive lateral faces at 90.degree. C. of its spacer grids. The clamping of the assemblies against the bearing faces of the cradle is ensured by flanges arranged level with each of the spacer grids of the assemblies.
In the prior art, level with each of the spacer grids of each of the fuel assemblies in the transport position on the cradle, two half-flanges are mounted in an articulated manner on the cradle so as to be capable of being turned down in the direction of the fuel assembly and of the supporting surfaces when the assembly is being fastened. The half-flanges in the turned-down position are connected to one another by means of a screw connection. Each of the half-flanges carries a device for clamping the assembly, consisting of a threaded rod engaged in the corresponding half-flange and carrying a bearing shoe at its end. The clamping of the assembly is ensured by bringing the shoes to bear with some pressure on the outer lateral surfaces of the spacer grids opposite the faces of these spacer grids bearing on the cradle, by means of a nut and a lock nut which are engaged on the threaded rod.
To ensure the clamping and flanging of the assembly on the cradle, it is therefore necessary to turn down each of the two half-flanges, to ensure the assembling together of the two half-flanges by clamping the screw connection and to bring the shoes to bear by torque-clamping each of the nuts.
These operations, which must be carried out for each of the spacer grids of the fuel assemblies, are relatively time-consuming and require the attendance of well-trained personnel, insofar as a defective fastening of the assemblies occurring as a result of insufficient clamping can result in damage to the assemblies during their transport. Likewise, excessive clamping of the shoes can cause damage to the spacer grids of the assemblies.
Moreover, the use of wrenches for clamping the screw connections and the nuts of the shoes entails some risk that these wrenches, which are of large mass, will fall onto the fuel assemblies and that the rods of the assemblies will therefore be damaged. Falling can also cause physical injuries to the operators responsible for putting the fuel assemblies into the container.
It is clear that the clamping devices according to the prior art also have similar disadvantages when the fuel assemblies are unloaded after a container has been opened. In fact, it is necessary to unclamp the shoes and the flanges before the cradle and the assemblies have been raised vertically, for example by the use of a mechanical device for raising the underframes.
In more general terms, in many sectors of industry, it is necessary to transport heavy components inside containers or on transport underframes, after effective clamping or flanging of the components.
This clamping of the components on their transport underframe is generally carried out by the use of screw connections fastened to a flanging part of the structure opposite the bearing surfaces of the object to be transported.
The clamping torque of these screw connections has to be carefully controlled.
The corresponding operations are therefore relatively time-consuming and difficult and usually require the attendance of highly competent personnel.